1. Field of the Invention
The present invention relates to electronics, and, in particular, to Power Sourcing Equipment (PSE) for communication systems conforming to the IEEE 802.3 Ethernet and IEEE 802.3af Power over Ethernet (PoE) standards.
2. Description of the Related Art
FIG. 1 shows a block diagram of a portion of a conventional Ethernet switch 100 for a communication system that conforms to both the IEEE 802.3 Ethernet standard and the IEEE 802.3af PoE standard, the teachings of both of which are incorporated herein by reference. As shown in FIG. 1, for each port in the switch, Ethernet switch 100 comprises Ethernet Physical-Layer (PHY) module 102, RJ-45 Ethernet connector 104, 48-volt switcher 106, line-side PSE control and power conditioning module 108, isolated-side PSE control module 110, four-pair signal-isolation transformer 112, power-isolation transformer 114, and optical isolator 116.
According to the IEEE 802.3 Ethernet standard, Ethernet PHY module 102 communicates with a Media Access Controller (MAC) and switching function and processes incoming and outgoing differential data signals that are transmitted over an Ethernet cable (not shown) that is connected to switch 100 at Ethernet connector 104. According to the IEEE 802.3af PoE standard, switch 100 is a PSE-capable switch that can provide a 48-volt DC (cable power) signal on the Ethernet cable to power a so-called powered device (PD), such as an Internet Protocol (IP) telephone, such that the PD device does not require any additional power source to operate. The 48-volt DC signal is provided by switcher 106 and modules 108 and 110.
Transformers 112 and 114 and optical isolator 116 provide high-voltage electrical isolation between (1) circuitry, such as Ethernet PHY module 102, switcher 106, and PSE control module 110, located on the so-called isolated side of switch 100 (i.e., the primary side of the transformers) and (2) circuitry, such as Ethernet connector 104 and PSE control and power conditioning module 108, located on the so-called line side of switch 100 (i.e., the transformers' secondary side), to protect the isolated-side circuitry from high voltages that might appear on the line side, such as those that can occur when lightening strikes near an Ethernet cable connected to connector 104.
Switcher 106 provides an AC power signal that is converted by power-isolation transformer 114 into a transformed AC power signal that can be converted by line-side PSE control and power conditioning module 108 into the 48-volt (differential) DC signal that is applied to the center taps of the secondary-side coils of two of the four transformer pairs in signal-isolation transformer 112.
In the embodiment shown in FIG. 1, signal-isolation transformer 112 has four pairs of transformer coils, where the 48-volt DC signal is applied to the center taps of two of the four secondary-side coils. The IEEE 802.3 Ethernet standard also covers lower-rate (e.g., 10 Mbit and 100 Mbit) Ethernet systems that have only two pairs of coils, where two of the four wire pairs in the Ethernet cable are spares. According to the IEEE 802.3af PoE standard, the 48-volt DC signal can be applied directed to the two spare wire pairs to power a PD device. Although this specification describes Ethernet switches with ports having four pairs of transformer coils, the teachings of this specification apply equally well to Ethernet switches with ports having only two pairs of transformer coils and two spare wire pairs.
According to one conventional implementation, line-side PSE control and power conditioning module 108 provides two PSE control functions (i.e., detection and classification) and three PSE power conditioning functions (i.e., rectification, filtering, and impedance control).
During detection, line-side PSE module 108 sequentially applies two different, low-power signals to the transformer coils (either low-current or low-voltage depending on the implementation, such as a first low-power signal of approximately 3 volts followed by a second low-power signal of approximately 8 volts, instead of the full 48-volt DC signal) to enable line-side PSE module 108 to detect whether a valid PD device (which is required by the PoE standard to have a 25-Kohm impedance across the lines that provide the detection signal) is currently connected to Ethernet connector 104 via an Ethernet cable.
If such a PD device is detected, then line-side PSE module 108 (optionally) performs classification, during which line-side PSE module 108 increases the level of the applied signal to approximately 18 volts and measures the PD signature current draw to determine the power classification of the detected PD device.
If a PD device is detected and (optionally) classified, then line-side PSE module 108 performs power conditioning, during which line-side PSE module 108 generates and applies the appropriate 48-volt DC signal via connector 104 to the Ethernet cable to power the PD device. This power conditioning function involves rectification and filtering of the transformed AC signal that is applied to line-side PSE module 108 from the secondary side of transformer 114 to generate the 48-volt DC signal. The power conditioning function also involves impedance control for the 48-volt DC signal. If no PD device is detected, then line-side PSE module 108 does not generate and apply a 48-volt DC signal to the Ethernet cable.
Depending on the particular implementation, switch 100 may be configured to support multiple Ethernet ports, like the port associated with Ethernet connector 104. According to one such conventional implementation, line-side PSE control and power conditioning module 108 is implemented as a relatively large integrated circuit (IC) that is capable of simultaneously supporting four different Ethernet ports. Examples of such line-side PSE control and power conditioning modules are:                The 12-channel PoE Manager, Product No. PD64012, sold by PowerDsine Ltd. of Israel;        The Quad Integrated Power Sourcing Equipment Power Manager, Product No. TPS2384, sold by Texas Instruments Incorporated of Dallas, Tex.; and        The Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection module, Product No. LTC4258, sold by Linear Technology Corporation of Milpitas, Calif.        
These conventional PSE control and power conditioning modules provide a certain (relatively limited) level of operating functionality. In certain conventional Ethernet switches, such as switch 100 of FIG. 1, additional (e.g., switch-vendor value-added) functions are provided by isolated-side PSE control module 110, which is typically implemented using a microcontroller. One exemplary additional function that may be provided by isolated-side PSE control module 110 is power balancing between multiple (e.g., as many as 48 or more) PD devices connected to a single Ethernet switch, where power balancing is based on the results of the PD power classification performed on each PD device. To implement this additional operating functionality, isolated-side PSE control module 110 receives (explicit) information about the status of the operations at line-side PSE module 108 via optical isolator 116 and transmits control signals to control the operations at line-side PSE module 108 via optical isolator 116.
Such a conventional configuration for switch 100 has a number of disadvantages. First of all, there is a considerable dollar cost to providing all of this circuitry associated with these functions. Furthermore, when two or more PD devices are connected to different Ethernet connectors supported by a single line-side PSE control and power conditioning module, there is no electrical isolation to protect the rest of the PD devices from a lightening strike near any one of the PD devices.